The present invention relates to a method for manufacturing a cell structure suitable for a catalyst carrier.
At present, as a catalyst body for purifying automobile exhaust gas, there is used a catalyst body prepared by carrying a noble metal such as platinum (Pt), palladium (Pd), and Rhodium (Rh) as a catalyst component on a honeycomb-shaped ceramic catalyst carrier, which is a kind of porous cell structure (see, e.g., JP-A-2003-205246 and JP-A-2003-33664).
In order to manufacture such a catalyst body, it is important to set up and adjust carrying conditions so that the amount of a catalyst component carried may be appropriate. When the amount of a catalyst component carried is too little, sufficient exhaust gas purification performance or durability cannot be obtained. On the other hand, when the amount of catalyst component carried is excessive, the catalyst component such as Pt, Pd, and Rh, which are expensive and rare noble metals, is wasted to increase production costs.
A catalyst component is carried on a porous cell structure generally by a wash-coat method. Specifically, first, catalyst slurry containing a catalyst component and water is filled into a cell structure. Then, water in the catalyst slurry is absorbed by the pores of the partition walls, and the catalyst component, which is a solid component in the catalyst slurry, adheres to a surface of the partition walls and the inside of the pores opening on a surface of the partition walls by the water-absorbing power. At this time, the amount of the catalyst carrier carried can be considered to be in proportion to the water-absorption amount of the cell structure.
In order to carry a catalyst component on a plurality of cell structures, one is arbitrarily selected from the cell structures, and conditions for wash-coating are set up on the basis of partition wall thickness, pore properties, and the like, of the selected cell structure. Then, a plurality of cell structures are uniformly subjected to wash-coating according to the conditions set up above to carry a catalyst component on the cell structures.
However, a plurality of cell structures have differences in partition wall thickness, pore properties, and the like. Therefore, there is a problem that carrying of a catalyst component on all the cell structures under the wash-coat conditions set up on the basis of partition wall thickness and the like of the arbitrarily selected cell structure is prone to cause variance in amount of the catalyst component carried.
It is considered that variance in amount of the catalyst component carried can be avoided by making partition wall thickness and the like of all the cell structures the same. However, an extrusion molding die used to form a ceramic raw material into a honeycomb structure gradually wears down as the number of extrusion molding increases. Therefore, partition wall thickness of the resultant cell structure tends to gradually increase as the number of extrusion molding increases. Since it is impossible to completely eliminate wear of an extrusion molding die, difference in partition wall thickness of the cell structures obtained cannot be avoided as a matter of fact.